The start node is the first node in a workflow. It exists by default when you create a new workflow and looks like the following image.
In case there is no Trigger node in the workflow, the workflow always starts from the Start node. The Start node cannot be deleted. Even if a workflow contains a Trigger node, there would still be a Start node.
# When is it necessary to use the Start node?
When using the Execute Workflow node in workflow A to execute workflow B, the Start node will act as the trigger node in workflow B. The second node in workflow B needs to be connected to the Start node for the workflow to execute correctly.